Modern wind turbines are commonly used to supply electricity into the electrical grid. Wind turbines generally comprise a rotor with a rotor hub and a plurality of blades. The rotor is set into rotation under the influence of the wind on the blades. The rotation of the rotor shaft drives the generator rotor either directly (“directly driven”) or through the use of a gearbox. The gearbox (if present), the generator and other systems are usually mounted in a nacelle on top of a wind turbine tower.
The loads on the rotor may be controlled by pitching the blades, i.e. by rotating each blade around its longitudinal axis (from root to tip) which causes the wind to engage a larger or smaller surface of the blade. Lift and drag can thus be changed to influence the aerodynamic torque on the rotor. This way, even though the wind speed may increase, the torque transmitted by the rotor to the generator can remain substantially the same.
For the purposes of pitching the blades, each blade may be mounted on the hub employing a pitch system. In many known turbines, such a pitch system may comprise a pitch bearing arranged between the hub and the blade and a pitch drive which may include an hydraulic or electric motor. The motor, through the use of a reduction gearing (sometimes referred to as a “reductor”, or as a “reduction gear”) drives an actuating gear.
In these implementations, planetary gearboxes are often used. Planetary gearboxes are generally able to deliver high reduction ratios in small packages, and also to transmit several times more torque than similarly sized, conventional gear units. Furthermore, they are compact and lightweight, and require little installation space.
The actuating gear (pinion) may generally be arranged to mesh with an annular gear provided on the wind turbine blade to set the blade into rotation and change its pitch angle. It is also possible however, to provide the annular gear on the hub, whereas the motor and actuator may be mounted on the blade.
It is also known to provide pitch configurations with a central gearbox.
In these central pitch configurations, one flange is normally connected between the hub and the gearbox and another flange or disc is connected between an output of the gearbox and the blade. Documents EP2253840 and ES1074081 describe such systems.
Several disadvantages related to more classical electro-mechanical pitch systems related e.g. to fretting corrosion resulting in premature wear of certain annular gear teeth repeatedly touching each other i.e. those teeth contacting each other in the pitch position at or below nominal wind speed can be avoided with a central pitch configuration. However, the constantly varying torque around the blade's longitudinal axis and bending loads on the blades may cause misalignments, vibrations and bending loads in the planetary gearbox internal mechanisms such as bearings and gears. The lifetime of such components (or the system as a whole) may thus be shortened.
It is an object of the present disclosure to provide blade pitch systems that at least partially reduces one or more of the aforementioned drawbacks.